1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus useful in the exploration for hydrocarbons located in subsurface formations. More particularly, the invention relates to the use of tubulars, such as casing, and drilling with such casing using a top drive.
2. Description of the Related Art
In the construction of oil and gas wells, it is usually necessary to line the borehole with a string of tubulars, known as casing, which are sequentially threaded together and lowered down a previously drilled borehole. Because of the length of the casing required, sections or stands of two or more individual lengths of casing are progressively added to the string as it is lowered into the well from a drilling platform. To add additional lengths of casing to that already in the borehole, the casing already lowered into the borehole is typically restrained from falling into the well by using a spider located in the floor of the drilling platform. The casing to be added is then moved from a rack to a position above the exposed top of the casing situated in the spider. The threaded pin (male threaded section) of this section or stand of casing to be connected is then lowered over the threaded box (female threaded section) of the end of the casing extending from the well, and the casing to be added is connected to the existing casing in the borehole by rotation therebetween. An elevator is then connected to the top of the new section or stand and the whole casing string is lifted slightly to enable the slips of the spider to be released. The whole casing string, including the added length(s) of casing, is lowered into the borehole until the top of the uppermost section of casing is adjacent to the spider whereupon the slips of the spider are reapplied, the elevator is disconnected and the process repeated.
It is common practice to use a power tong to torque the connection up to a predetermined torque in order to make the connection. The power tong is located on the platform, either on rails, or hung from a derrick on a chain. However, it has recently been proposed to use a top drive for making such connection. A top drive is a top driven rotational system used to rotate the drill string for drilling purposes.
It is also known to use the casing, which is typically only lowered into the borehole after a drill string and drill bit(s) have been used to create the borehole, to actually drive the drill bit to create the borehole, thereby eliminating the need to remove the drill string and then lower the casing into the borehole. This process results in a substantial increase in productivity since the drill string is never removed from the borehole during drilling. To enable this efficiency, the casing is cemented in place once each drill bit or drill shoe reaches its desired or capable depth, and a new drill bit and casing string are lowered through the existing casing to continue drilling into the earth formation. The borehole can be drilled to the desired depth by repeating this pattern.
The use of casing as the rotational drive element to rotate the drill shoe or drill bit in situ has revealed several limitations inherent in the structure of the casing as well as the methodologies used to load and drive the casing. For example, the thread form used in casing connections is more fragile than the connection used in drill pipe, and the casing connections have to remain fluid and pressure tight once the drilling process has been completed. Additionally, casing typically has a thinner wall and is less robust than drill pipe. This is especially true in the thread area at both ends of the casing where there is a corresponding reduction in section area. Furthermore, casing is not manufactured or supplied to the same tolerances as drill string, and thus the actual diameters and the wall thicknesses of the casing may vary from lot to lot of casing. Despite these limitations, casing is being used to drill boreholes effectively.
It is known in the industry to use top drive systems to rotate a casing string to form a borehole. However, in order to drill with casing, most existing top drives require a crossover adapter to connect to the casing. This is because the quill of the top drive is not sized to connect with the threads of the casing. The quill of the top drive is typically designed to connect to a drill pipe, which has a smaller outer diameter than a casing. The crossover adapter is design to alleviate this problem. Typically, one end of the crossover adapter is designed to connect with the quill, while the other end is designed to connect with the casing.
However, the process of connecting and disconnecting a casing is time consuming. For example, each time a new casing is added, the casing string must be disconnected from the crossover adapter. Thereafter, the crossover adapter must be threaded into the new casing before the casing string may be run. Furthermore, this process also increases the likelihood of damage to the threads, thereby increasing the potential for downtime.
More recently, top drive adapters have been developed to facilitate the casing handling operations and to impart torque from the top drive to the casing. Generally, top drive adapters are equipped with gripping members to grippingly engage the casing string to transmit torque applied from the top drive to the casing. Top drive adapters may include an external gripping device such as a torque head or an internal gripping device such as a spear.
The spear typically includes a series of parallel circumferential wickers that grip the casing to help impart rotational or torsional loading thereto. Torque is transferred from the top drive to the spear. Typically, the spear is inserted into the interior of the uppermost length of the string of casing, engaged against the inner circumference of the casing, and turned to rotate the string of casing and drill shoe in the borehole.
When a spear is used for drilling with casing (DWC), the spear is known to damage the interior surfaces of the casing, thereby resulting in raised sharp edges as well as plastic deformation of the casing caused by excessive radial loading of the spear. Scarring or other sources of sharp raised edges interfere with the completion of, and production from, the well formed by the borehole, because rubber, plastic and other readily torn or cut materials are often positioned down the casing to affect the completion and production phases of well life. Further, the ultimate strength of the individual casing joint deformed is reduced if the casing undergoes plastic deformation, and the casing joint may later fail by rupture as it is being used downhole during or after drilling operations. Finally, it is known that the load necessary to grip a string of casing in a well may result in rupture of the casing.
Therefore, there exists a need for a drilling system which enables make up of casing and drilling with casing following make up. Preferably, the drilling system can accommodate variable sizes and weights of casing without causing deformation or rupture of the casing.